The Czochralski method (CZ method) involves pulling up a growing single crystal ingot from a raw material melt, for instance of silicon, in a crucible. In order to appropriately control the growth of the single crystal, the liquid level (hereafter, melt level) of the raw material melt must be accurately measured, and the position thereof must be adjusted according to the growth of the single crystal.
In particular, silicon single crystal pulling apparatuses (CZ furnaces) using the CZ method are ordinarily provided with a heat shield for controlling the heat radiation from a heater and the silicon melt, and for straightening the flow of gas infused into the CZ furnace.
The thermal history and the impurity concentration (for instance, oxygen concentration) in the pulled single crystal can be kept constant by controlling the relative position (i.e. the distance) between the lower face of the heat shield and the melt level.
Various conventional melt level measuring methods have been proposed to this end. The method for measuring melt level of the invention of the present application employs reflection beams. Except where misunderstanding might arise, the measuring method will be referred to as “reflection method.”
Japanese Patent Application Laid-open No. 2000-264779 a method for measuring melt level by triangulation, wherein the melt surface in the crucible is regarded as a direct reflection body. This measuring method will be referred to hereafter as “direct reflection method.”
FIG. 18 is a diagram for explaining the trajectory of a laser beam in a direct reflection method. FIG. 18A is a schematic diagram of the trajectory of a laser beam as viewed laterally (X-Y plane). FIG. 18B is a schematic diagram of the trajectory of a laser beam as viewed from the front (X-Z plane). In FIG. 18A, the laser beam is guided by a rotating mirror 9 and a prism 11. In FIG. 18B, the rotating mirror 9 and the prism 11 are omitted, since the trajectory of the laser beam in the Y-axis direction is perpendicular to the paper. To simplify the explanation, portions not essential to triangulation will be omitted.
In FIGS. 18A and 18B, a silicon raw material 3 is melted inside a crucible 2 provided in a CZ furnace 1. A silicon single crystal 4 is pulled up and grown, while rotating toward the upper portion of the figure. A heat shield 5 is disposed outside the silicon single crystal 4. Herein, D denotes the gap between the peripheral wall of the silicon single crystal 4 and the inner peripheral face (side face 5b) of a rim 5a provided at the lower end of the heat shield 5, while L denotes the gap between a melt surface 7 and a lower face 6 of the rim 5a provided at the lower end of the heat shield 5.
In the above invention, a range-finding unit 8 working on the principle of triangulation is used for measuring the melt level of the melt surface 7.
In FIG. 18B, the range-finding unit 8 has provided therein a laser beam source 12 that projects a laser beam and a photodetector 13 that receives a reflected return beam. A lens 13a that condenses the incident laser beam and a linear CCD sensor 13b that detects the condensed laser beam are provided in the photodetector 13.
The laser beam emitted by the range-finding unit 8 is reflected on the rotating mirror 9, passes through an entrance window 10 and a prism 11 disposed in the CZ furnace 1, and strikes the melt surface 7.
The projection position of the laser beam on the melt surface 7 is scanned in the radial direction of the crucible 3 (arrow S2 in the figure) through left-right rotation of the rotating mirror 9 (arrow S1 in the figure). As a result, a return beam reflected on the melt surface 7 passes through the prism 11, the entrance window 10 and the rotating mirror 9, and is received by the photodetector at a predetermined measurement frequency (number of measurements per unit time). In the direct reflection method, thus, the laser beam emitted by the laser beam source is projected directly onto the melt surface 7, and the return beam reflected on the melt surface 7 is received directly by the photodetector 13.
When the melt level of the melt surface 7 is positioned at a position A1, the laser beam reflected on the melt surface 7 is detected at a measurement coordinate X1 in the linear CCD sensor 13b. That is, the measurement coordinate X1 of the linear CCD sensor 13b corresponds to the melt level A1. Likewise, when the melt level is positioned at a position A2, the laser beam reflected on the melt surface 7 is detected at a measurement coordinate X2 in the linear CCD sensor 13b. That is, the measurement coordinate X2 of the linear CCD sensor 13b corresponds to the melt level A2. The melt level can thus be worked out, by triangulation, from the measurement coordinates detected by the linear CCD sensor 13b. 
The incidence angle and the reflection angle (both angle θ1) of the laser beam on the melt surface 7 have been exaggerated in the figure. In actuality, the angle θ1 is small, of several degrees. The same is true in other instances.
Patent document 2: WO 01/083859 discloses a method for measuring melt level by causing a laser beam emitted by a laser beam source to be scattered once on the lower face of a heat shield, and to be reflected twice on a melt surface. This measuring method will be referred to hereafter as “return reflection method”.
FIG. 19 is a diagram for explaining the trajectory of a laser beam in the return reflection method.
FIG. 19A is a schematic diagram of the trajectory of a laser beam as viewed laterally (X-Y plane). FIG. 19B is a schematic diagram of the trajectory of a laser beam as viewed from the front (X-Z plane). In FIG. 19A, the laser beam is guided by the rotating mirror 9 and the prism 11. In FIG. 19B, the rotating mirror 9 and the prism 11 are omitted, since the trajectory of the laser beam in the Y-axis direction is perpendicular to the paper. To simplify the explanation, portions not essential to triangulation will be omitted.
In FIG. 19B, the range-finding unit 8 has provided therein a laser beam source 12 that emits a laser beam and a photodetector 13 that receives a reflected return beam. A lens 13a that condenses the incident laser beam and a linear CCD sensor 13b that detects the condensed laser beam are provided in the photodetector 13.
In FIGS. 19A and B, a laser beam emitted by the laser beam source 12 is reflected by the rotating mirror 9 and the prism 11 and is projected onto the melt surface 7. The projected laser beam is reflected on the melt surface 7 (melt level A4), and the reflection beam is projected onto the lower face 6 of the rim 5a of the heat shield 5 provided above the melt surface 7. The projected laser beam is scattered at a scattering point 6a on the lower face 6 of the heat shield, and the scattered scatter beam is projected again onto the melt surface 7. The projected laser beam is reflected again on the melt surface 7, and the resulting reflection beam is finally received by the photodetector 13.
That is, the laser beam received by the photodetector 13 is the reflection beam of the laser beam projected onto the melt surface 7 from the scattering point 6a of the lower face 6 of the rim 5a. As viewed from the photodetector 13, the laser beam is detected as being emitted from a scattering point 3a on an apparent reflection surface.
In FIG. 19B, the incidence angle and the reflection angle of the laser beam in the X-Z plane have the same value at all times. In simple geometrical terms, the scattering point 3a on the apparent reflection plane and the scattering point 6a on the lower face 6 of the rim 5a of the heat shield 5 have a fold-back positional relationship (specular relationship) relative to the actual melt level A4. The apparent reflection plane will be referred to hereafter as melt level A3. To better grasp the apparent laser beam trajectory, the position of an apparent heat shield 5c, resulting from specular replication of the heat shield 5 at the actual melt level A4, is depicted in broken lines in FIG. 19B. Therefore, the gap between the lower face 6 of the rim 5a and the apparent melt level A3 is 2L.
The position of the lower face 6 of the rim 5a can be determined by measuring, for instance, the position of the top face 9 of the rim 5a. In FIG. 19, the position of a reflection point 9a of the laser beam at the top face 9 of the rim 5a corresponds to a measurement coordinate X9 of the linear sensor. If a distance M between the top face 9 and the lower face 6 is measured beforehand, the position of the lower face 6 of the rim 5a can be determined from the measurement coordinate X9 and the distance M.
The gap L can be determined as half the value of the relative distance 2L between the position of the lower face 6 of the rim 5a and the apparent melt level A3. The actual melt level A4 can be determined as a value standing above the melt level A3 by the gap L.
An inclined portion including a surface-tension meniscus (hereinafter, “inclined portion”) is formed at the site where the outer wall of the pulled single crystal comes into contact with the melt surface. The inclination angle increases in the vicinity of the outer wall of the single crystal. Also, the entire melt surface exhibits a paraboloid shape resulting from the rotation of the crucible and the rotation of the pulled single crystal. When the heat shield, which straightens gas flow, stands close to the melt surface, the melt surface close to the underside of the heat shield may take on a concave shape on account of the discharge pressure of infused gas. The inclination of the melt surface shifts the angle of the reflection beam of the laser beam, and hampers stable detection of the beam. The shape of the inclined portion varies depending on the manufacturing conditions, and hence the shape of the inclined portion must be actually measured or estimated in accordance with the manufacturing conditions.
The direct reflection method has a drawback in that the gap D decreases when there is set a large diameter of the pulled single crystal, and in consequence, the laser beam is reflected on a large-inclination spot. The reflection direction of the laser beam shifts as a result, so that the reflected laser beam may fail to return to the photodetector at predetermined measurement frequencies, in which case the probability of receiving the reflection beam (hereafter, “beam reception probability”) becomes zero.
On the other hand, the direct reflection method is advantageous in that the melt surface 7 is used as a direct reflection body, so that the distance L can be measured irrespective of whether it is large or small. Moreover, the return beam is a direct reflection beam from the melt surface 7, and hence laser power may be small.
The return reflection method is advantageous in that it affords a comparatively high beam reception probability, even when the gap D is small, since the method utilizes a scatter beam from the lower face 6 of the rim 5a. 
On the other hand, the return reflection method has a drawback in that the intensity of the scatter beam scattered on the lower face 6 of the rim 5a is weak, and the laser beam is reflected twice on the melt surface 7. As a result, the intensity of the laser beam ultimately received by the photodetector is weak. That is, the return reflection method requires greater laser power. Also, reducing the value of the gap L in order to control the quality of the single crystal gives rise to a lower beam reception probability, on account of the structure by which the laser beam is scattered at the lower face 6 of the rim 5a. 
As described above, an inclined portion forms in the vicinity of the site at which the outer wall of the pulled growing single crystal comes into contact with the melt surface. The reflection site on the melt surface is different in the direct reflection method and the return reflection method, and hence the influence exerted by the inclined portion on beam reception probability is likewise different.
FIG. 20 is a diagram for explaining the influence of the inclined portion on the direct reflection method and the return reflection method.
In FIG. 20, the reflection site is set on the melt surface 7, in the vicinity of the inner diameter of the heat shield 5, for the return reflection method. For the direct reflection method, by contrast, the reflection site is set on the melt surface 7 shifted by a predetermined distance from the inner diameter of the rim 5a towards the center of the crucible. In the direct reflection method, therefore, the reflection site is closer to the outer wall of the single crystal than is the case in return reflection method. The influence of the inclined portion is therefore greater in the direct reflection method. Moreover, the influence of the inclined portion becomes more significant as the outer diameter of the pulled single crystal increases.
In the return reflection method, the reflection site is set in the vicinity of the inner diameter of the rim 5a, and hence the influence of the inclined portion is less than in the case of the direct reflection method. Nevertheless, the influence of the inclined portion cannot be neglected when the outer diameter of the pulled silicon single crystal increases and the gap D narrows.
As described above, both the direct reflection method and return reflection method have advantages and drawbacks. It is therefore not easy for an operator to decide which reflection method to use according to the manufacturing conditions.
In the light of the circumstances above, it is an object of the invention of the present application to provide a method that allows measuring a liquid level, reliably and easily, by selecting an optimal reflection method, from among a plurality of reflection methods, in accordance with the growth conditions of a pulled single crystal.